Aromatic polyether ketones have excellent heat resistance, chemical resistance, molding processability, mechanical properties and electric properties, and widely used in electronic parts, jigs for semiconductor process, mechanical parts of automobiles and OA equipments.
Methods of producing an aromatic polyether ketone are roughly classified into two polymerization routes. One is an aromatic electrophilic substitution reaction, and another is an aromatic nucleophilic substitution reaction. As the aromatic electrophilic substitution reaction, there are, for example, a method of obtaining a polyether ketone by using nitrobenzene as a solvent (see U.S. Pat. No. 3,065,205), and a report of slight increase in molecular weight by using dichloromethane as a solvent (UK Patent No. 971,227). However, any of them give extremely low molecular weight.
U.S. Pat. No. 3,442,857 discloses that a polyether ketone having high molecular weight is obtained by an aromatic electrophilic substitution reaction, in a HF/BF3 system by using a liquid hydrogen fluoride as a solvent. However, in the method of producing a polyether ketone by an aromatic electrophilic substitution reaction, a straight chain polymer is not obtained easily because a polymer having high molecular weight is not obtain easily and the reaction position on an aromatic ring cannot be restricted easily, and resultantly, a thermal physical property is poor. Even obtained, it is necessary to use a strong acidic solvent of high toxicity, therefore, processes such as neutralization and deoxidation treatments become inevitable, and they are not preferable in industrial view points.
Furthermore, it is also known that a polyether ketone having low molecular weight is obtained by direct dehydration polycondensation by an aromatic electrophilic substitution reaction using a polyphosphoric acid as a solvent (see J. Polym. Sci. A-1, 6, 3345 (1968)). However, a phosphorus component remains as an impurity in the resulted polyether ketone because polyphosphoric acid is used in this system. Thus it is not preferable.
UK Patent No. 1,387,303 discloses that a polymer having high molecular weight is obtained likewise, even if HF/BF3 is directly used in a dehydration polycondensation method. However, it is necessary to use a strong acidic solvent of high toxicity. Thus it is not preferable in industrial use.
On the other hand, it is known that polymerization of a polyether ketone by an aromatic nucleophilic substitution reaction can be conducted by producing an alkali metal salt of an aromatic diol using an alkali metal compound as a catalyst, and then effecting a polycondensation reaction with a halogenated benzophenone compound (see, J. Polym. Sci. A-1, 5, 2375 (1967)). The above-mentioned aromatic nucleophilic substitution reaction is called a desalting polycondensation method because a halogenated alkali metal salt is released in polycondensation. Usually, in this method, polymerization progresses by reaction of a polymer dissolved in a solvent. However, a polymer having low solubility in a solvent such as, for example, a polyether ether ketone deposits at the stage of low molecular weight and the subsequent polymerization reaction does not progress. Therefore, a polymer having high molecular weight cannot be obtained.
It is said important to sufficiently remove water from the reaction system in a desalting polycondensation method wherein the water is by-produced in production of an alkali salt of an aromatic diol. For example, Japanese Patent Application Publication (JP-B) No. 42-7799 discloses that it is essential to keep a solvent under anhydrous condition before the reaction and during the reaction in a polymerization reaction. For removing water from the reaction system, a method of distilling off by benzene or toluene as an azeotropic solvent of water and a method of removing the distilled water by adsorption on a molecular sieve are used.
Regarding the polyether ether ketone having low solubility in a solvent, the polymer having high molecular weight is obtained by keeping the reaction system wherein the polymer is constantly dissolved at extremely high temperature. However, in this method, it is necessary to conduct a polymerization reaction at extremely high temperature, and an extremely complicated process is necessary for purification. Macromolecules, 23, 4029 (1990) discloses that when diphenylsulfone is used as a solvent in production of polyether ether ketone, the molecular weight is not increased at polymerization temperatures of not more than 280° C. at which the produced polymer deposits. Similarly, there is also a method in which diphenylsulfone is used as a polymerization solvent, temperature is gradually increased for keeping the produced polymer in dissolved condition, and the reaction is effected finally at 320° C. (see JP-B No. 60-32642). As described above, in conventional methods of producing polyether ether ketone, it has been necessary to conduct a polymerization reaction at extremely high temperatures for obtaining a high molecular weight polymer.
However, because the reaction is conducted at extremely high temperature for the solution polymerization as described above, there is a fear of gelation and decomposition reaction of a polymer to enlarge the molecular weight distribution, and additionally, economical and environmental aspects are also poor. Furthermore, when the reaction liquid is cooled after completion of the reaction, the produced polymer is coagulated together with a catalyst and an alkali metal salt, therefore, a process of fine grinding is necessary for purification of a polymer.
Furthermore, it is extremely difficult to finely mechanical-grind a polymer having high molecular weight which is sufficient to obtain an excellent mechanical strength as a molded article and coating, and it has been impossible to obtain a polymer having an alkali metal content of 20 ppm or less even if washing is reinforced so much.
Under the above-mentioned conditions, there has been a necessity for a polyether ketone having various physical properties such as molecular weight and mechanical physical properties equivalent to those of conventional products, having a polymer particle size smaller sufficiently than that of conventional products, and having a content of impurities such as alkali metal components smaller sufficiently than that of conventional products, and a method of producing the same.